The present novel concept broadly relates to fluid suspension systems and, more particularly, to an air spring assembly having an improved piston construction and a method of manufacturing the same.
It is to be specifically understood that the subject novel concept is capable of broad use in a wide variety of suitable applications and environments and can be used in association with air spring assemblies of any suitable size, type and/or configuration without departing from the principles thereof.
One category of known air spring assemblies, referred to in the art as rolling lobe-type air springs, typically includes a top plate, a piston and a flexible sleeve secured therebetween. The flexible sleeve forms a lobe that rolls up and down an outer side wall of the piston in response to loads applied to the top plate and/or piston. In such assemblies, the piston is normally formed from either a metal material, typically steel, or a plastic material. Each construction has numerous benefits as well as some disadvantages, and the selection of one construction versus the other will vary from application-to-application.
One benefit of producing a piston from plastic is that the piston can often be injection molded as a complete or nearly complete component. As a result, costs associated with physically assembling the piston can be significantly reduced or eliminated. Additionally, it is well understood that air springs are commonly exposed to harsh environments, such as in vehicle suspension applications in which water, dirt, salt and other materials are present. Another benefit is that pistons formed from a plastic material are often less susceptible to exposure of this kind.
There are, however, numerous disadvantages associated with manufacturing a piston from a plastic material. One disadvantage is that air spring piston applications typically require high performance plastic materials. These types of specialty plastics provide advanced material properties (e.g., high strength, high impact resistance, low water absorption, UV resistance) over common commodity grade plastics. Accordingly, specialty plastics are normally considerably more expensive than common commodity grades. For smaller sized pistons, significant cost savings can be achieved by molding a plastic piston as a complete part and eliminating any associated assembly costs. However, as the size of the piston increases, the quantity of material used to form the piston likewise increases, in many cases at greater than a 1:1 ratio. Therefore, the cost associated with the manufacture of larger pistons can significantly increase due, at least in part, to the increased expenses of the additional quantity of the plastic material. Thus, in some cases, particularly in the case of bigger sizes, the assembly cost savings can be largely offset by the increased material costs. As such, at some point it becomes less expensive to use metal and incur the associated assembly costs.
Another disadvantage of manufacturing pistons from a plastic material is associated with the method of manufacturing the parts. That is, pistons formed from a plastic material are typically injection molded. The injection molding process imposes certain design constraints on the configuration of the parts being produced. For example, it is generally understood that it is desirable for the wall thicknesses of the molded part to be substantially uniform. This can help to minimize undesirable part shrinkage and can also assist in promoting material flow into the mold cavity. In practice, this is a real constraint that limits the options available in designing the part or component.
Another example of a design constraint attendant to the injection molding process is due to the action of retracting a portion of the mold to eject the finished part. It is well understood that the mold halves that make up a mold cavity are pulled apart so that the molded part, once cured, can be removed or ejected. For this action to be completed without damaging the part, there must be no parts of the mold tooling embedded in or otherwise interengaging the plastic part. Otherwise, this piece of the mold tooling will either damage the part when the mold halves are separated or prevent the separation thereof altogether. This makes it difficult to mold features into a part that extend lateral to the direction of the mold pull. Thus, this too is a design constraint that limits the options available in designing the part or component. In some cases, secondary operations can be utilized to machine or otherwise produce such features. However, this increases the cost of producing the part and offsets the cost benefit of molding the part complete.
Pistons formed from a metal material, typically steel, are also commonly produced. Like the plastic pistons discussed above, metal pistons have numerous advantages and disadvantages. Some of the significant advantages include the strength properties associated with metals, such as steel, for example. The ultimate strength of steel is considerably greater than the strength of most plastic materials. Additionally, steel and other metals have elastic yield properties that are absent in plastic materials. Thus, a metal piston of an air spring assembly is capable of yielding under a load, whereas a plastic piston might become permanently deflected.
Unfortunately, the significant assembly cost savings associated with molded parts is often not available where a corresponding part is manufactured from metal. This is primarily due to the fact that metal components cannot be manufactured complete in the same manner as molded parts. Rather, numerous metal pieces are often assembled into a completed part. Thus, significant assembly costs are often associated with the manufacture items from metal materials, and these costs are typically considerably higher than the corresponding material costs.
However, metal material costs can also add significantly to the overall production costs of a finished part, such as a piston for an air spring assembly, particularly in known designs that utilize thick materials as well as in comparatively large pistons. Thus, as metal material costs increase, the overall production costs also increase, and these increases can be significant in some cases.
Accordingly, an improved air spring assembly and method have been developed that overcome these and other disadvantages.